Laser processing systems commonly coordinate the firing of laser pulses to positions on a workpiece by means of a trigger signal that is generated by the beam-positioning system and delivered to the laser control electronics as an external input. In such methodologies, the beam-positioning system can be considered to be the “master” and the laser is considered to be the “slave” because the timing decisions about when to fire the laser are wholly computed and determined by the beam-positioning system. The trigger signal from the beam-positioning system causes the laser control electronics to activate a pulse initiation device, such as a Q-switch, to cause a laser pulse to be emitted.
Some of these positioning-based machining systems strive to minimize pulse energy variability with an “off by default” approach, wherein laser pulses are only fired when the beam positioner arrives at target locations. The low laser power, nature of this process, and system architecture result in a relatively low duty cycle of laser pulses in such systems. In some wafer-dicing laser-machining systems, a galvanometer controller arbitrarily sends trigger signals to a Q-switch to initiate laser pulse emission when the beam axis directed by the galvanometer mirrors arrives at the desired position at a known velocity.
The problem with the above-described “laser follows beam positioner” scenario is that lasers perform best, and their pulse parameters are more consistent, when the lasers are operated at a steady state. Moreover, any non-constant starting, stopping, or frequency changes in timing of the trigger signal pulse train sent to the Q-switch will result in undesirable transients in the laser output beam, such as initial hot pulses, low-frequency average power drift, and increased pulse-to-pulse pulse width variability and peak power variability. These variations reduce control of laser parameters applied to the work surface, thereby reducing processing window, and may degrade laser-processing quality. U.S. Pat. No. 6,172,325 of Brian Baird et al. and U.S. Pat. No. 7.61,669 of Keith Grant et al. describe improvements in beam position-based laser-synchronization methods in which the beam-positioning system is the master. These patents are both assigned to the assignee of this application and are both incorporated herein by reference.